This invention relates to a control of a torque converter clutch and more particularly to a system for regulating the torque capacity of the clutch to control the torque converter slippage.
Introduced as an efficiency increasing device, the torque converter clutch is a fluid operated friction device engageable to couple the impeller (input) and turbine (output) of a hydraulic torque converter. In the usual application, the clutch is either fully released to permit unrestrained slippage between the impeller and the turbine or fully engaged to prevent such slippage entirely. An unfortunate aspect of full converter clutch engagement is that the engine torque perturbations or torsionals normally absorbed by the torque converter are passed directly through the clutch to the vehicle drivetrain and may produce annoying pulsations therein if not properly damped. This factor operates to restrict the usage of the torque converter clutch to specified vehicle operating conditions for which the annoying effects are minimized. As a result, the potential efficiency gains afforded by engagement of the torque converter clutch have only been realized over a portion of the range of vehicle operation.
To overcome the disadvantages of torque converter clutch engagement, it has been proposed to operate the clutch in a slipping mode wherein a predetermined amount of slippage between the torque converter impeller and turbine is permitted for regulating the torque capacity of the clutch. In any such system, the objective is to isolate engine torque perturbations in the torque converter while passing steady state engine torque at a slip rate that provides improved torque converter efficiency. One such system that controls the clutch slippage to achieve the above objectives is disclosed in U.S. Pat. No. 4,582,185 to Grimes et al., issued Apr. 15, 1986 and assigned to the assignee of the present invention.
Generally speaking, the system identified above operates to generate clutch engagement force without regard to the magnitude of the slip error. Whenever the measured slip is greater than the desired slip, the controller acts to increase the clutch engagement force to increase the torque capacity of the clutch. Whenever the measured slip is less than the desired slip, the controller acts to decrease the clutch engagement force to decrease the torque capacity of the clutch.
While the control of the torque converter clutch in a slip mode may be desirable over a significant range of various driving conditions, further refinements to the torque converter clutch control can be made to improve overall efficiency. For example, full torque converter clutch engagement whereby slip is substantially zero may be acceptable during periods of vehicle operation where torque disturbances are not expected to be objectionable. Typically, highway cruising with nominal vehicle loading and sufficient engine speed may benefit from such full engagement of the torque converter clutch. However, as loading and engine speed vary, torque disturbances may become objectionable and release of full engagement desirable. Both the application and release of a torque converter clutch into or out of a full engagement may themselves become objectionable particularly where engine load or speed conditions vary.